1. Field of the Invention:
This invention relates to cameras, and more particularly to cameras having electromagnetic drive sources.
2. Description of the Prior Art:
Cameras recently have been proposed including shutter curtains which are directly driven by electromagnetic drive sources such as motors, i.e., electrically operated magnet control cameras. Since the shutter is directly driven by an electromagnetic force produced from an electrically operated magnet drive source, that electromagnetic force must always be maintained at a constant magnitude. Otherwise, the shutter time cannot be accurately controlled. In other words, the speed of movement of the shutter curtain is determined by the magnitude of the output of the electromagnetic drive source. Therefore, as the electromagnetic force changes, the running speed of the shutter curtains will also change. This makes it impossible to accurately control the shutter time.
In the case of relatively long shutter times, the above problem is not very noticeable owing to the fact that the period of shutter actuation is long. However, with faster shutter times, the problem is quite serious because the running time error of the shutter curtain largely affects the overall exposure time. Therefore, it is common practice to provide a constant current circuit or other suitable constant electrical power supply means, in order to insure that a constant electromagnetic force can always be obtained from the shutter drive source. Even with such a constant current circuit, however, when the input voltage drops below a predetermined level, a constant output supply current cannot be achieved, and the above-described problem persists.
Another problem with the electric operated magnetic control cameras, which arises from the use of two electromagnetic drive sources to separately control the operation of the leading and trailing curtains of the shutter, is that when the shutter time is so small that the periods of actuation of the leading and trailing curtains partially overlap each other, the electrical energy consumed during the overlapping period becomes large. Therefore, when the shutter time is set at such a small value and the battery voltage is low, not only is an incorrect exposure time provided, but, as the battery voltage drops to less than a satisfactory operating level for the electromagnetic drive sources as the shutter is running, the shutter curtain stops from moving further thereby leaving the exposure aperture partly open.